ChEMBL

We know that man (and women) can actually live on potatoes alone, but there is a lurking danger in the humble spud, and due to poor housekeeping on my part, I managed to transform man's starchy friend into a toxic time-bomb - enough of the dramatics, I left some potatoes in the daylight on top of my microwave and they went green. And as all children are taught from an early age, green potatoes are 'poisonous'. This toxic effect is mediated by a number of compounds, but the major one is Solanine - the toxic effects include gastrointestinal, cardiac and neurological effects. Another toxic component of potatoes is the closely related compound Chaconine, and tomatos contain yet another related toxic compound Tomatine. Doses of 3 to 6 mg per kg of Solanine can be fatal to a human (so for an average 70 kg human, 210-420 mg of Solanine). A 'green potato' can have up to 1000 mg of Solanine per kg, so it is relatively easy to ingest enough potato to elicit a toxic response.

The chemistry and biology behind this is quite interesting, and basically it is a defence mechanism against fungi, insects, slugs, and other potato predators, and this includes us - since we are now the super-predator for the potato species (Solanum tuberosum). The exposed portions (stems, leaves, berries, etc) of the potato plant have high levels of Solanine - in fact potatoe plants are in the same family as the Deadly Nightshade (Atropa belladonna) of North European folklore (itself a cornucopia of useful and toxic natural products). Normally the tubers of potato plants are buried, hidden from light, and have low levels of Solanine. When harvested they are typically kept in light proof sacks, or in the dark of a larder. When exposed to light, the potato responds by kicking off photosynthesis (and inducing chlorophyll production to achieve this, this is the green colour in the skin). Solanine is not green, but it's production tracks that of chlorophyll - therefore the green colour is a useful biomarker of potato toxicity.




Solanine is a complex steroidal glycoalkaloid that can cause apoptosis in a broad range of cells (believed to be via a Bcl-2 apoptosis pathway, since solanine reduces the expression of Bcl-2 in HepG2 cells (an oft used human cell line for toxicity testing). Solanine has a broad range of pharmacology ranging from disrupting lipid membrane structure and stability, there is a strong affinity of Solanine for cholesterol (an essential membrane component) leading to cell lysis; Solanine also inhibits butyryl cholinesterase - and thereby affects neurotransmission. The chemical structure of Solanine contains a large fused-ring basic alkaloid, coupled to three sugar rings. These sugar rings will hydrolyse in the acid of the stomach, and so oral ingestion is less toxic than other delivery routes. Also Solanine is lipophilic and will therefore partition into fats (one more reason to enjoy chips!).

Finally the potato genome is being determined, and so the biosynthetic genes for Solanine will be in there. The PlantCyc resource has a useful overview of the biosynthesis of Solanine - which is derived from the same precursor (mevalonic acid) used in the synthesis of cholesterol.  As you would expect due to the immense commercial importance of the potato, this field has attracted some patenting interest e.g. US 7439419.